Page 39 - Mike's published articles

M. J. Murphy & P. A. Torstensson
Received: 20 August 2013 / Accepted: 16 September 2013 # Springer-Verlag London 2013
Abstract Thermal relaxation times were introduced into modern skin-laser science with the inception of selective photothermolysis. As a result, laser pulsewidths were deter- mined according to the thermal relaxation times of the tissue targets. The Arrhenius Damage Integral shows that this ap- proach is incorrect. The important parameter is the time re- quired to induce irreversible protein denaturation within the target. This time is determined by the tissue’s intrinsic struc- ture, not its physical dimensions. This report explains why thermal relaxation times should not be considered when treating many skin conditions with lasers or IPL systems.
Keywords Photothermal . Relaxation time . Arrhenius equation . Protein denaturation . Lasers . IPL . Vessels
Introduction
Selective photothermolysis (SPT) has been the cornerstone of many modern-day dermatology laser procedures since its inception in 1981 [1, 2]. This theory was constructed at a time when short-pulsed lasers were being developed for vascular conditions. Anderson and Parrish identified the problems with older technology and redefined the concept of acceptable clinical results following laser interventions.
A major part of SPT was the definition of thermal relaxa- tion time (TRT). This was used to determine the maximum
M. J. Murphy (*)
DermaLase Training Services, Glasgow, UK e-mail: mike.murphy@virgin.net
P. A. Torstensson
Photonova of Sweden AB, V Frolunda, SE e-mail: pa.torstensson@photonova.com
allowable pulsewidths to be used on blood vessels while minimising thermal injury to adjacent tissue. The TRT was defined as the time taken ‘for the central temperature of a Gaussian temperature distribution with a width equal to the target’s diameter to decrease by 50 %’ and is calculated (in cylinders as a first approximation) as follows:
TRT 1⁄4 d2=16 α ð1Þ
Where d is the target diameter (in millimetre) and α is the tissue diffusivity (in square millimetre per second). The theory of SPT proposes that minimal collateral thermal damage is achievable if the laser energy is delivered within this time.
Hence, a pulse of energy is applied to the skin surface with a duration calculated according to the diameter of the target tissue. When considering relaxation times, Anderson and Par- rish were solely concerned with minimising collateral thermal damage to the dermis surrounding the target blood vessels.
This definition was extended by Altshuler et al. [3] in 2001 to include hair follicles, under the same principles as SPT, whereby target structures are denatured or destroyed by heat energy diffusing from adjacent light energy-absorbing chro- mophores in the hair shaft (absorber) rather than by direct absorption in the outer root sheet (target). To their surprise, they found experimentally that selective photothermal damage of hair follicles occurred even for radiant pulse durations five to seven times longer than the TRT of the hair follicle, which they could not explain. Haemoglobin and melanin are the light-absorbing targets in blood vessels and hair, respectively. Their temperatures become elevated and these heat sources conduct thermal energy to the target tissues—the vessel walls and hair germ cells [3]. The authors defined a new time—the ‘thermal damage time’ as the time when the outermost part of the target reaches the target damage temperature through heat diffusion from the heater (tissue), but this heat diffusion model
Author's personal copy
Lasers Med Sci
DOI 10.1007/s10103-013-1445-8
ORIGINAL ARTICLE
Thermal relaxation times: an outdated concept in photothermal treatments